A rapid and sensitive method to determine the levels of human immunodeficiency virus type 1 (HIV-1) in plasma from patients with HIV-1 infection was established by using RNA polymerase chain reaction (PCR). Our plasma RNA-PCR technique quantitatively detected HIV-1 particles in plasma from 76 of 77 (98.7%) HIV-1 infected individuals examined at varying clinical stages with a range of more than 4 orders of magnitude, while p24 antigen capture enzyme-linked immunosorbent assay (ELISA) failed to detect circulating p24 antigen in 43 of 72 (59.7%) seropositive individuals. Acid treatment prior to ELISA increased the positivity for p24 antigen, but 21 of 60 (35.0%) seropositive individuals still remained negative. The numbers of HIV-1 particles in plasma from patients with AIDS or ARC were markedly higher than those in plasma from asymptomatic seropositive individuals (p<0,0001). When the changes of viral load following the antiviral therapy were monitored by using this plasma RNA PCR method, 10 of 10 patients who received oral 2',3'-dideoxyinosine (ddI) at > 6.4 mg/kg/day doses had a significant decrease in plasma HIV-1 particle numbers following 8 to 14 weeks of therapy (p=0.0051). These data suggest that plasma HIV-1 virion levels determined by the current RNA PCR technique represent the actual plasma HIV-1 viremia status in patients with varying stages of HIV-1 infection, and may provide clinically useful information, especially in patients with negative serum p24 antigen. However, more research is required to evaluate the usefulness of this technique in assessing the disease status and monitoring the effect of anti-retroviral therapy.